The present invention relates to mixers for superheterodyne communication receivers such as television receivers. More particularly, the invention relates to an improved integrated mixer simultaneously exhibiting, over a relatively wide band of frequencies, a low conversion loss and a high signal handling capability.
It is known that conventional superheterodyne communication receivers require a stage which converts or translates an input signal frequency to an intermediate frequency for subsequent processing by the receiver. For example, in a television receiver a local oscillator signal is combined with the received RF television signal in a mixer stage to produce the intermediate frequency (IF) signal which is coupled to the IF section of the receiver. Typically, a single ended diode mixer is used for this purpose. In such a device both the local oscillator signal and the received RF television signal are injected into a single port comprising one or both terminals of a diode. In addition the resulting IF signal is also developed at this single port. The three signals developed at the port are normally decoupled from each other by means of suitable filter circuits connected between the port and the source of the local oscillator signal and the received RF signal as well as the IF signal utilization apparatus. Although mixers of the foregoing type have been traditionally used in television receivers, their performance from both conversion loss and RF signal handling capability viewpoints is considered to be rather poor. In other words, single ended diode mixers of this type provide only minimal rejection of distortion products produced by the mixer.
The single balanced mixer was devised in an effort to overcome some of the problems characterizing single ended diode mixers. An exemplary prior art single balanced mixer is shown in Hewlett Packard Application Note 965 and comprises a two port device, one port constituting the input of a balanced transformer whose secondary winding includes a center tap connected to ground potential. The local oscillator signal is injected through one port comprising the primary winding of the balanced transformer while the RF signal is injected and the IF signal obtained at the second port comprising the junction between two mixing diodes connected across the secondary winding. As in the case of the single ended diode mixer, suitable filter circuits are required to decouple the RF and IF signals. Although the single balanced mixer is characterized by increased distortion product rejection, it nevertheless exhibits a relatively high conversion loss and low RF signal handling capabilities.
The double balanced mixer evinces a prior art attempt to further improve on the performance of the single ended diode mixer. A double balanced mixer, also shown in the foregoing application note, comprises three ports and is characterized by the use of first and second balanced transformers. Four diodes are normally connected between the two secondary windings of the transformers, the secondary winding of the first transformer having a center tap connected to a ground potential. The local oscillator signal is injected into one port constituting the primary winding of the first transformer while the RF signal is injected at a second port constituting the primary winding of the second transformer. The IF signal is developed at a third port comprising a center tap connected to the secondary winding of the second transformer. While providing improved RF/IF decoupling and also achieving increased distortion product rejection, the conversion loss and RF signal handling capabilities characterizing prior art double balanced mixer are not significantly improved over those of the single balanced mixer.
It is therefore the primary object of the present invention to provide a novel mixer arrangement simultaneously exhibiting a relatively low, wide band conversion loss and a high RF signal handling capability.
It is a further object of the invention to provide an improved mixer having the foregoing attributes and also being conveniently manufacturable at a low cost.